1. Field of the Invention
This invention relates in general to a method and apparatus for use to prevent actuation of a downhole well tool until the downhole well tool is downhole within a wellbore, and in particular to a heat activated safety fuse for use in a wireline pressure setting assembly to prevent actuation of the wireline pressure setting assembly until after it is lowered downhole within a wellbore.
2. Background of the Invention
Prior art downhole well tools include electrically operated downhole well tools which present hazards to both wellsite operators, and wellsite equipment and operations, if they are operated at the ground surface of the wellbore. A few examples of such downhole well tools are those which use explosive materials to either provide explosive forces to perform work within a wellbore, such as perforating guns, tubing cutters, and back-off shots. Another example of such downhole well tools are squibs and solenoids, which are for releasing mechanical members within wellbores. Still another example are downhole well logging tools which release radiation or radioactive materials. Yet another example is a wireline pressure setting assembly for converting a chemical source into a force over a distance for performing work in setting packers, bridge plugs, and similar devices within wellbores.
For example, one such prior art downhole well tool is a cable conveyed bridge plug for setting within a cased wellbore such as that shown in U.S. Pat. No. 2,637,402, entitled "Pressure Operated Well Apparatus," invented by R. C. Baker et al., and issued to Baker Oil Tools, Inc. on May 5, 1953. A similar cable conveyed downhole well tool is disclosed in U.S. Pat. No. 2,695,064, entitled "Well Packer Apparatus," invented by T. M. Ragan et al., and issued to Baker Oil Tools, Inc. on Nov. 23, 1954. These patents disclose cable conveyed downhole well tools for setting a bridge plug within a wellbore casing. These cable conveyed downhole well tools were actuated by the percussion of a firing pin causing a cartridge to explode and ignite a prior art power cartridge, or combustible charge.
Another example of a prior art wireline conveyed well packer apparatus is disclosed in U.S. Pat. No. Re. 25,846, entitled "Well Packer Apparatus," invented by D. G. Campbell, and issued to Baker Oil Tools, Inc. on Apr. 31, 1965. The wireline conveyed well packer apparatus disclosed includes a power charge which is ignited to generate gas for setting the well packer apparatus within a wellbore. The power charge is ignited by passing an electric current down the wireline and exploding an igniter cartridge, which causes a flame to ignite the power charge.
An example of a prior art power charge for use in downhole well tools to generate a gas to provide a force is a combustion charge disclosed in U.S. Pat. No. 2,640,547, entitled "Gas-Operated Well Apparatus," invented by R. C. Baker et al., and issued to Baker Oil Tools, Inc. on Jun. 2, 1953. The combustion charge is comprised of combustion materials which, when ignited within a downhole well tool disclosed in the patent, will take at least one second for a maximum pressure to be attained within the downhole well tool. This prior art combustion charge includes both a fuel and a self-contained oxygen source. The combustion charge is ignited to generate a gas having a pressure which provides a force for setting the gas-operated well apparatus. The combustion charge of the gas-operated well apparatus is ignited by exploding an igniter to start the combustion reaction for burning the combustion charge. The combustion charge, once ignited, burns in a self-sustained combustion reaction to generate the gas.
A prior art wireline pressure setting assembly disclosed in U.S. Pat. No. 2,692,023, entitled "Pressure Operated Subsurface Well Apparatus," invented by M. B. Conrad, and issued to Baker Oil Tools, Inc. on Oct. 19, 1954. This wireline conveyed downhole well tool includes a power charge which is burned in a combustion reaction to generate a gas. The power charge is ignited by electrically exploding an igniter cartridge which then emits a flame to start the power charge burning. Combustion of the power charge generates the gas having a pressure which provides force for operation of the wireline setting tool to set a downhole tool such as a packer or bridge plug within the wellbore.
Each of the above-referenced patents, U.S. Pat. No. 2,640,547, U.S. Pat. No. Re. 25,846, U.S. Pat. No. 2,695,064, U.S. Pat. No. 2,637,402, and U.S. Pat. No. 2,692,023, are hereby incorporated by reference as if fully set forth and disclosed herein.
The above prior art downhole well tools for converting the chemical components of a power charge into a mechanical force exerted over a distance typically require a separate igniter cartridge for igniting the power charge. Additionally, other pyrotechnic wellbore devices utilize an igniter, as well as incorporate an igniter into a singular package with the pyrotechnic device. Typically, explosive components are used for prior art igniter materials, such as, for example, gunpowder or lead azide. These types of igniter materials are easily ignited and represent hazards both to operators utilizing these materials in downhole well tools, and to successful completion of wellsite operations. Some of these types of primary ignition or igniter materials are susceptible to ignition from applications of small amounts of electric current, or even discharge of static electricity.
Wellsite operations utilizing prior art downhole well tools which present hazards if operated outside of the wellbore would be safer if prevented from operating until lowered downhole within a wellbore. Such a safety feature would enhance operator safety, as well as promote successful wellsite operations.